Hypertension involves elevated vascular resistance and only exists in a living animal, where the physiologic regulators of vascular tone (i.e. neuronal activity, blood flow, and endocrine factors) are intact. Therefore, hypertension research will greatly benefit from the in vivo exploration of the molecular regulators of arterial smooth muscle cell contraction. Our overall goal is to elucidate mechanisms of increased vascular resistance, for the first time, in conscious animals, during experimental salt-dependent hypertension. The use of conscious animals (as opposed to anesthetized) is key to understanding the putative role of sympathetic nerve activity (SNA), increasingly recognized as a key mechanism of salt-dependent hypertension. This overall goal is to be achieved through the use of non-invasive, fluorescence imaging of molecular signaling in arterioles of conscious optical biosensor mice. Given that phosphorylation of myosin regulatory light chains is the critical determinant of smooth muscle contraction, Specific Aims 1 and 2 are to develop optical biosensor mice that express novel, genetically-encoded activity biosensors for the key molecular regulators of smooth muscle myosin phosphorylation; a) myosin light chain kinase, (MLCK), b) myosin light chain phosphatase, MLCP), and c) a key upstream regulator of MLCP, the small GTPase, RhoA. In Aim 3, we will use these optical biosensor mice to determine, in vivo, 1) the regulation of MLCK, MLCP and RhoA by certain vascular G-protein coupled receptors (GPCR) putatively involved in hypertension, including adrenoceptors (?1-AR), Angiotensin II receptors (AT1-R), endothelin-1 receptors, (ETA), and sphingosine-1-phosphate receptors (S1P1). The regulation of MLCK, MLCP and RhoA by SNA will be determined through the use of complete autonomic ganglionic blockade (hexamethonium) in conscious mice. Mice will be implanted with telemetric arterial blood pressure transducers to allow continuous measurement of arterial blood pressure during imaging (and all other times). In Specific Aim 4, the time course of the activation levels of MLCK, MLCP and RhoA will be measured in ear arterioles of conscious individual mice (i.e. a `longitudinal' study) during 14 days of Angiotensin II/salt hypertension. Mice are infused chronically with Angiotensin II and fed a high-salt (NaCl) diet. Increased vascular resistance in this model of salt-dependent hypertension is believed to involve heightened SNA (`sympathoexcitation) emanating from salt-sensitive CNS cardiovascular control regions, mandating use of conscious mice. These Specific Aims will be performed in this multi-PI project under the direction of two Principal Investigators with the necessary expertise to generate the proposed sensors (Rizzo) and perform the physiologic experiments (Wier). In summary, we expect to achieve dynamic imaging of myosin phosphorylation regulatory processes during the development of salt-sensitive hypertension in a living animal for the first time. These studies will reveal new insights on the molecular basis of increased vascular resistance in hypertension, as it actually occurs in living animals.